Republicans call for Biden to resign as they attack Kamala Harris

Mr Biden’s rival for president, Donald Trump, said the Democratic leader was “not fit to serve from the very beginning” in response to the announcement – though he did not call for the president to resign.

Senator Steve Daines of Montana, who chairs Senate Republicans’ campaign arm, said that being president “is the hardest job in the world”.

“And I no longer have confidence that Joe Biden can effectively execute his duties as Commander-in-Chief,” he said in a statement.

Another Republican senator, Markwayne Mullin of Oklahoma, went further and appeared to suggest that Mr Biden should be forced from office by exercising the 25th Amendment of the US Constitution – a never-used method to replace the president if he cannot fulfil his duties.

Critics of Trump had called for using the amendment to remove him when he was in office.

Many in the political world had been expecting to Mr Biden to drop out of the race.

His rambling, frequently incoherent answers in the June 27 debate with Trump had stunned the country and left people wondering if he could serve as president for another four years. While in speeches and interviews Mr Biden often showed renewed vigour, he was also dogged by major stumbles and seeming memory problems.

Democrats in congress, worried that his shakiness would hurt their chances at re-election, and major donors began to press for him to drop out, but they did not press for him to resign.

The last president to abandon his election campaign, Lyndon B. Johnson, also served out the remainder of his term. Like Mr Biden, Johnson had said that giving up the race would allow him to focus on his presidential duties.

As the pressure on Mr Biden has grown in recent weeks, Republicans became more vocal about a resignation.

Just hours before the president announced he was stepping aside, Trump’s new running mate, Senator JD Vance of Ohio, said: “Not running for reelection would be a clear admission that President Trump was right all along about Biden not being mentally fit enough to serve as Commander-in-Chief. There is no middle ground.”

“Joe Biden has been the worst President in my lifetime and Kamala Harris has been right there with him every step of the way,” he added.